Paperboard tubes are widely used in the paper, film and textile industries to wind material as it is manufactured. The paperboard tubes themselves are manufactured continuously by spirally winding multiple paperboard strips, or plies, around a stationary mandrel. Paperboard tubes are also made in a convolute winding process by forming a multiple layer wrap of a wide paperboard sheet around a stationary mandrel. Although paperboard is relatively weak on a single ply basis, a tube constructed from multiple spirally wound plies or convolute wound layers of paperboard can attain substantial strength.
In recent years, paperboard tube winding cores have been subjected to increasingly higher levels of stress, due to changes in film and fiber properties, improvements in winders, and changes in package sizes. In the textile industry, substantial increases have been seen in the strength of various yarns, such as multi-filament continuous yarns of nylon, polyester, etc., resulting in the application of increased compressive force to the tube exterior. In the film industry, improved materials and processes have also resulted in higher winding tensions and increased stress on film winding cores. At the same time, efficiency considerations and improvements in automation have resulted in increased quantities of yarn and film wound onto individual yarn and film packages, further increasing the compressive forces applied to the paperboard winding cores.
These increasing compressive forces have increased the occurrences of tube "failure" of the type commonly known as inside diameter (ID) "comedown", which involves a decrease in the tube ID during the winding process. In many textile and film winding processes, the winding core is supported on a winding mandrel. In the event of substantial winding core ID comedown during the winding process, the paperboard core forming the interior of the finished yarn or film package, can so tightly grip the exterior surface of a winding mandrel that the final package cannot be removed from the winding mandrel until the wound yarn or film has been removed from the core, typically by cutting, thus destroying the yarn or film.
It is generally understood that the overall strength of paperboard tubes can be increased by increasing tube wall thickness and/or by employing stronger paper strips for the plies of the tube. In this regard, paper is available in a wide variety of strengths. Paper strength is improved by increasing the mechanical refining of paperboard pulps and by compressing the paperboard during manufacture. Further, paperboard strength is influenced by fiber type and quality. As a general rule, stronger paperboard sheets have a higher density than low strength paperboard sheets.
However, increasing the wall strength of paperboard winding cores by increasing the wall thickness of the cores increases use of natural resources, i.e., wood pulp. And forming the tube walls from higher density, higher strength papers can result in greater use of energy resources due to the greater energy resources often used in forming these papers. Moreover, the nature of the high strength, high density papers limits the types and amounts of recycled paper that can be used in the paper manufacturing process.
In response to industry needs for stronger paperboard cores, substantial effort has been focused on tube manufacturing processes and tube designs. Paperboard is an orthotropic material. Thus, paperboard strength properties are different in the machine direction (MD) and in the cross machine direction (CD) (MD refers to the direction of paper production during the manufacturing process, and CD refers to the direction perpendicular to the MD in the plane of the paper). The difference in properties between the MD and CD can be attributed to the tendency for more paper fibers to be aligned along the MD as compared to the CD. The orthotropic properties of paper influence tube strength and complicate any accurate prediction of tube strength.
In addition, the paperboard strips used to prepare spirally wound paperboard tubes are wound at varying angles, and tube properties depend, at least in part, on the winding angle of the spirally wound strips. The winding angle thus further increases the difficulty of accurately predicting paperboard tube properties.
As with other materials, paperboard tubes exhibit different strength values depending on which strength characteristics are measured. These different strength characteristics, such as compressive strength, tensile strength, beam strength, etc., can vary according to tube construction. The standard industry test to evaluate the strength of paper tubes is the flat crush test. This test involves compressing a tube along its sides by placing the tube between two flat plates. One plate is stationary while the other moves at a constant displacement rate transversely to the axis of the tube. The flat crush strength is the maximum load obtained during the test. The flat crush test has been relied on in the past as an indicator of a tube's resistance to inside diameter reduction, i.e., ID comedown, during a winding process.
Radial crush strength of paperboard tubes can also be evaluated by applying increasing fluid pressure loads uniformly around the circumference of the tubes until their failure; see T. D. Gerhardt, External Pressure Loading of Spiral Paper Tubes: Theory and Experiment, Journal of Engineering Materials and Technology, Vol. 112, pp. 144-150, (1990). This paper also provides a detailed mechanics analysis of stresses and strains in single-grade spirally wound paperboard tubes loaded in uniform radial compression and concludes, inter alia, that the maximum hoop stress occurs at the outside radius of spirally wound paperboard tubes under these conditions.
Although paperboard tubes are typically manufactured primarily from single paper grades, multi-grade configurations are also used for various reasons. For example, in some cases, a special surface finish is needed on the tube outside diameter (OD) or on the tube ID, and a paper ply having such a finish is therefore used on the OD or ID. Different grades of paper are also used in order to satisfy other special property requirements for the tube ID or OD, for example, as might be required for interaction with a chuck or other structure.
Although multiple grades of paper have previously been used to produce spirally wound paperboard tubes, multiple grades of paperboard have not in the past, to the knowledge of the present inventors, been positioned with the intent and effect of minimizing inside diameter reduction during a winding process involving a large radial compression loading.